The invention relates to sewing machines in general, and more particularly to improvements in mechanisms for regulating or controlling the needle thread between a spool or another suitable source and the eye of the needle. Still more particularly, the invention relates to improvements in mechanisms wherein the needle thread is acted upon by resilient means to be maintained under requisite tension.
When the needle of a sewing machine descends toward and penetrates into the fabric on the lower arm of a sewing machine, that length of needle thread which is located ahead of the eye of the needle must be controlled to prevent the development of short-lasting pronounced tensional stresses such as could adversely influence the making of stitches. To this end, it is customary to employ an oscillatory takeup lever which is movable up and down in substantial synchronism with the needle in such a way that the length of needle thread ahead of the eye of the needle cannot undergo an abrupt and pronounced tensioning action and is also prevented from becoming too loose so that it could form a relatively large loop which is likely to be wound around the needle.
A drawback of presently known takeup levers for the needle thread ahead of the needle is that they cannot accurately control the thread during each and every stage of a stitch forming step. Therefore, it is customary to employ a spring which compensates for the inability of the lever to control the needle thread during certain stages of each stitch forming operation. In addition, such spring is called upon to perform one or more additional functions in the course of a stitchforming step, depending upon the nature of the takeup lever or another takeup member. Thus, when the takeup member moves downwardly toward the fabric on the lower arm of the sewing machine, the needle thread tends to become loose and the spring is called upon to temporarily take up the thread until the eye of the needle penetrates into the fabric. This is desirable and advantageous because the spring ensures that the needle cannot penetrate into loose needle thread, that the thread cannot be wound around the lower end of the needle, and that the thread cannot become twisted immediately ahead of the eye in the lower end portion of the needle. This reduces the likelihood that the thread would break and/or that an overly loose or overly taut needle thread could interfere with the making of satisfactory stitches.
The spring is called upon to perform a different function shortly before the looptaker casts a freshly formed and enlarged needle thread loop. At such time, the spring ensures gentle and shock-free withdrawal of the needle thread loop before the takeup member becomes active to rise at an elevated speed and to pull the needle thread upwardly and into the fabric.
Still further, the spring can be used to ensure that the looped thread is withdrawn at the instant when the finger of the bobbin and the rotation stopping member (which interrupts the rotational movement of the bobbin) are located exactly opposite each other. This ensures a gentle and noise-free withdrawal of the thread. As a rule, the pull of the spring upon the thread should not be very pronounced; however, the spring should be capable of rapidly altering the tension of the thread when such tensioning is desired and the tensioning action must be carried out without losses, such as slippage.
In view of the aforediscussed numerous important and advantageous functions of the spring, the mounting of such spring, the configuration and dimensions of the spring as well as the location of the spring are of considerable importance because they can greatly influence the quality of stitch making. In addition, it is desirable to locate the spring in such a way that it permits rapid, convenient and reliable insertion of the thread, i.e., the person in charge of using the sewing machine need not pay much attention to the spring during introduction of the thread into the prescribed path between the source of such thread and the eye of the needle.
U.S. Pat. No. 4,856,443 to Ogawa et al. discloses a needle thread tensioning device which is designed to enable a spring to maintain the needle thread under tension during a certain stage of each stitch forming step. The takeup member for needle thread is mounted in such a way that its thread engaging portion is adjacent the locus of passage of the needle thread loop along the member which is used to stop rotation of the bobbin. The purpose of the patented thread tensioning device is to ensure constant withdrawal of needle thread from the spool and to avoid the generation of pronounced noise. A drawback of the device of Ogawa et al. is that it cannot properly take up loose needle thread during the initial stage of downward movement of the needle and cannot prevent the accumulation of loose needle thread immediately adjacent the eye of the needle. Moreover, the construction which is proposed by Ogawa et al. is rather bulky, complex and expensive.
Commonly owned Swiss Pat. No. 421 673 to Gegauf discloses a needle thread regulating mechanism which employs a thread deflecting spring. A drawback of this proposal is that the regulating mechanism is quite complex and that the extent to which the thread is looped on the spring varies during different stages of a stitching step with attendant changes of friction between the spring and the thread.
A different thread regulating mechanism is disclosed in German Pat. No. 45813 to Gebr. Nothmann. This patent proposes to subject needle thread to an additional tensioning action prior to as well as in the course of a stitch forming step.
Other thread regulating mechanisms are disclosed in U.S. Pat. No. 4,422,395 to Singer and in U.S. Pat. No. 4,736,698 to Brother.